Implement `Client.battery();`

Returns `Client._lastBattery`.
Added calibrate command to UdpControl and Client, which according to
the SDK docs "asks the drone to calibrate the magnetometer (must be
flying)."
Merge pull request #26 from janl/battery-levels

Implement `Client.battery()`
Merge pull request #51 from wiseman/add_calibrate

Added calibrate command to UdpControl and Client